1. Field of the Invention
This invention relates to intrusion alarms. The invention relates more particularly to an improved ultra-sonic method and apparatus for detecting the presence of an intruder in a protected space or certain other environmental changes.
2. Description of the Prior Art
Intrusion alarms have been utilized for detecting the presence of an intruder in a protected area. In a known intrusion alarm system employing Doppler principles, a transmitter and receiver are located in, or adjacent to, an area to be protected and a continuous wave of ultra-sonic energy is broadcast by the transmitter at a predetermined frequency. Acoustical energy at the same frequency is reflected by stationary objects in the protected area and is sensed by the receiver. Insofar as the reflected energy occurs at the same frequency, the apparatus determines that an intruding condition does not exist and no alarm is sounded. The passage or movement of an intruder through the protected area, however, causes a variation in the frequency of the reflected energy which variation is sensed and an alarm is sounded.
This form of intrusion alarm system is recognized to be substantially susceptible to false alarms. Any movement in the protected area will cause a frequency shift causing a false alarm. The movement can be caused, for example, by the fluttering of a window curtain, the passage of a pet through a room and such other movements as can normally be expected to occur. Such false alarms are of course undesirable and reduce the reliability and value of the system.
An alternative pulse-echo technique for object detection is known wherein a pulse of acoustical energy is projected from a transducer and the occurrence of a reflected pulse within a predetermined time interval is indicative of the presence of an object in an area being examined. Prior pulse-echo object detecting apparatus, however, have been relatively complex and costly and do not readily lend themselves to use as intrusion alarm detectors.